An Enantiospecific Synthesis of 5-epi-α-Bulnesene

As a result of its unique fragrance and wider role in traditional medicine, agarwood produced in Aquilaria spp. and certain other trees has been harvested to near extinction as a natural phenomenon. Artificially induced agarwood production in Aquilaria plantations has sated some of the demand although the product quality is variable. Synthetic chemistry may have a role to play in providing sustainable routes to many of the fragrant components identified in agarwood and its smoke when burnt as incense. In this work, we report efforts towards a total synthesis of the guaiane sesquiterpene α-bulnesene, which is found, along with its more fragrant oxidised derivatives, in agarwood. Following the ring-expansion of (R)-carvone using reported procedures, α-butenylation gave a substrate for samarium diiodide mediated reductive cyclisation, the two butenyl epimers of the substrate each leading to a single bicyclic alcohol (24 and 25). Overall homoconjugate hydride reduction of one of these alcohols was achieved by Lewis acid-mediated ionisation and then hydride transfer from triethylsilane to complete an overall seven-step synthesis of 5-epi-α-bulnesene. This new synthesis paves the way for short routes to both α-bulnesene enantiomers and a study of their aerial and enzymatic oxidation products.


Introduction
A variety of tree species from, notably, the Aquilaria and Gyrinops genera (Thymelaeaceae) react to stress by the production of an oleoresin, known inter alia as aloes or agar, that darkens the tree's heartwood characteristically and progressively with time (years to decades) [1]. This dense, resinous heartwood-agarwood (oud, gahara, jinkoh and others)-has been revered for its unique fragrance for at least 3400 years. Burnt as incense or steam-distilled to form the essential oil, naturally occurring agarwood has become increasingly rare, exacerbated by a growing demand for agarwood objects [2]; the highest grades can sell to aficionados for USD 1000s per gram creating a vicious circle of spiralling demand and cost [3]. Some consequences of this include indiscriminate felling of potential agarwood-bearing trees, to the extent that South-East Asian Aquilaria and Gyrinops species are now protected under the Convention on International Trade in Endangered Species of Wild Fauna and Flora [4]; a black market of poached, smuggled, adulterated, or fake agarwood [5,6]; agarwood investment schemes (and scams) [7]; and the development of artificial methods of inducing agarwood production in Aquilaria plantations by physical, chemical, and biological (fungal inoculation) means [8][9][10][11][12].
The cultural importance, desirability, and high commercial value of agarwood stimulated extensive and ongoing research directed towards understanding the physiological and biochemical processes leading to its production [13] and identifying the chemical components chiefly responsible for the fragrance [14][15][16][17]. As expected, the number and distribution of secondary metabolites identified within different samples of agarwood-including the smoke from smouldering samples, the vapours of heated samples, and the essential including the smoke from smouldering samples, the vapours of heated samples, and the essential oil-vary significantly but, in general, somewhere between 200-400 compounds are present within which sesquiterpenoids and 2-(2-phenylethyl)chromanones dominate [14]. Descriptions of the "characteristic" agarwood fragrance differ, in part due to the variety of sources but also due to inherent subjectivity and social context; typical descriptions include "rich, sweet woody, wet wood-like" [18], "warm, unique balsamic notes with sandalwood-ambergris tonalities" [17], "gorgeous, mysterious and elegant Oriental odor" [19]. The principal contributors to these properties are the relatively volatile sesquiterpenoids, of which 182 are listed in a recent review [14], that span a range of structural types including, especially, eudesmanes, eremophilanes, and guaianes.
Our interest in this area stemmed from two observations. First, the oxidation products of -bulnesene 1 (Figure 1), present in agarwood essential oil and extracts, contribute to the complex fragrance properties; aldehyde 2 and lactone 3 are described in Naef's influential review [17] as "pleasant, -damascenone-like, woody, touch of camphor" and "powerful, long lasting, woody, sweet note", respectively. Zviely and Boix-Camps are more effusive: "a beautiful note of -damascenone; its woodiness […], almost having soul and spirit" and "its smell is very strong […], extremely long lasting […], clean, radiant, diffusive, bright, woody ambery", respectively [20]. Second, in our collaborative research on employing engineered P450BM3 enzymes in synthesis, we have noted the kinetic resolution of racemic terpenoid substrates [21,22]. Putting these together led us to consider: (a) would the enantiomers of -bulnesene be discriminated by the Oxford P450BM3 library and (b) would the enantiomers of sesquiterpenoids 2 and 3 have different odours? To answer these questions, a source of both enantiomers of -bulnesene was sought. he naturally occurring enantiomer of -bulnesene is conveniently available by extraction from patchouli oil [23,24] and has been synthesised [25][26][27][28]. Of the total syntheses only Heathcock's route [27] from (±)-Wieland-Miescher ketone could, in principle, be modified to access the separate -bulnesene enantiomers but, at around 18 steps overall we aimed to find a shorter route that could be applied equally to both enantiomers. In this report we summarise our study of a free radical cyclisation route to the 7,5-fused ring system leading to 5-epi--bulnesene in just seven steps from (R)-carvone.

Results and Discussion
The synthetic strategy was shaped by our experience in constructing the pyrrolizidine ring system of heliotridane by a 1,5-hydrogen atom transfer/5-exo-trig cyclisation [29] which, in turn, had roots in the original work of Parsons [30][31][32] and Curran [33,34] For -bulnesene, the initial target was identified as vinyl iodide 3 (Figure 2), a precursor to radical intermediates 4 and then 5. The regiochemical outcome of the final cyclisation step was not fully predictable, given the lack of close precedent, but C-C bond formation at the methyl-bearing allyl radical terminus was considered to be less favourable on purely steric grounds. The stereochemical outcome was also unclear but the precedent for analogous 5,5-and 6,5-fused ring-forming processes supported the preferred production of the desired endo-methyl configuration; see also [35]. The naturally occurring enantiomer of α-bulnesene is conveniently available by extraction from patchouli oil [23,24] and has been synthesised [25][26][27][28]. Of the total syntheses, only Heathcock's route [27] from (±)-Wieland-Miescher ketone could, in principle, be modified to access the separate α-bulnesene enantiomers but, at around 18 steps overall, we aimed to find a shorter route that could be applied equally to both enantiomers. In this report we summarise our study of a free radical cyclisation route to the 7,5-fused ring system leading to 5-epi-α-bulnesene in just seven steps from (R)-carvone.

Results and Discussion
The synthetic strategy was shaped by our experience in constructing the pyrrolizidine ring system of heliotridane by a 1,5-hydrogen atom transfer/5-exo-trig cyclisation [29] which, in turn, had roots in the original work of Parsons [30][31][32] and Curran [33,34]. For α-bulnesene, the initial target was identified as vinyl iodide 3 (Figure 2), a precursor to radical intermediates 4 and then 5. The regiochemical outcome of the final cyclisation step was not fully predictable, given the lack of close precedent, but C-C bond formation at the methyl-bearing allyl radical terminus was considered to be less favourable on purely steric grounds. The stereochemical outcome was also unclear but the precedent for analogous 5,5and 6,5-fused ring-forming processes supported the preferred production of the desired endo-methyl configuration; see also [35]. The key step in an initial proposal for the synthesis of -bulnesene and its diastereomers by homolytic 1,5-hydrogen atom transfer (HAT) and 5-exo-trig cyclisation.
A short route was developed (Scheme 1) to vinyl radical precursor 3 that began with ring expansion [36] of dihydro-(R)-carvone 6 to afford the analogue 7 as an inconsequential mixture of diastereomers. Regioselective enolisation [37] and silylation, then sulfonylation [38] of the regenerated lithium enolate afforded enol triflate 9 with high (98:2) regioselectivity. The route was completed by employing Fürstner's variant of the Kumada coupling [39,40] followed by nickel-catalysed hydroalumination and iodination [41]. The low yield in the Kumada coupling step may reflect a slow metal insertion into the C-OTf bond in enol triflate 9 which is co-planar with the adjacent methyl; the analogous reaction of the regioisomeric enol triflate (S1, Supplementary Materials) proceeded efficiently (76% yield) despite the volatility of the product. All attempts to effect the 1,5-HAT/5-exo-trig sequence from iodide 3, or its cycloheptene regiosomer (S3, Supplementary Materials), using a variety of conditions based on R3SnH and AIBN, returned complex product mixtures whose 1 H NMR spectra consistently showed resonances corresponding to RCH=CH2 [ 5.89-5.77 (1H, m), 5.01 (1H, dd, J 17.0, 2.0 Hz), 4.94 (1H, dd, J 10.5, 2.0 Hz)], indicating simple reduction of the C-I bond (→11) as a dominant pathway. A reaction with Bu3SnD was considered, in order to identify which of the 1,5-HAT or cyclisation steps was problematic, but this was discarded in light of the complex product profile and poor mass recovery; instead, an unambiguous route to the allylic radical 5 was sought. The ring-expanded derivatives 13 and 14 ( Figure  3) of (R)-carvone (and their enantiomers from (S)-carvone) are available on a multigram scale via efficient three-or four-step sequences. From these, enolate alkylation and carbonyl reduction would provide allylic alcohol 12 from which a variety of radical precursors could potentially be prepared. The key step in an initial proposal for the synthesis of α-bulnesene and its diastereomers by homolytic 1,5-hydrogen atom transfer (HAT) and 5-exo-trig cyclisation.
A short route was developed (Scheme 1) to vinyl radical precursor 3 that began with ring expansion [36] of dihydro-(R)-carvone 6 to afford the analogue 7 as an inconsequential mixture of diastereomers. Regioselective enolisation [37] and silylation, then sulfonylation [38] of the regenerated lithium enolate afforded enol triflate 9 with high (98:2) regioselectivity. The route was completed by employing Fürstner's variant of the Kumada coupling [39,40] followed by nickel-catalysed hydroalumination and iodination [41]. The low yield in the Kumada coupling step may reflect a slow metal insertion into the C-OTf bond in enol triflate 9 which is co-planar with the adjacent methyl; the analogous reaction of the regioisomeric enol triflate (S1, Supplementary Materials) proceeded efficiently (76% yield) despite the volatility of the product.  The key step in an initial proposal for the synthesis of -bulnesene and its diastereomers by homolytic 1,5-hydrogen atom transfer (HAT) and 5-exo-trig cyclisation.
A short route was developed (Scheme 1) to vinyl radical precursor 3 that began with ring expansion [36] of dihydro-(R)-carvone 6 to afford the analogue 7 as an inconsequential mixture of diastereomers. Regioselective enolisation [37] and silylation, then sulfonylation [38] of the regenerated lithium enolate afforded enol triflate 9 with high (98:2) regioselectivity. The route was completed by employing Fürstner's variant of the Kumada coupling [39,40] followed by nickel-catalysed hydroalumination and iodination [41]. The low yield in the Kumada coupling step may reflect a slow metal insertion into the C-OTf bond in enol triflate 9 which is co-planar with the adjacent methyl; the analogous reaction of the regioisomeric enol triflate (S1, Supplementary Materials) proceeded efficiently (76% yield) despite the volatility of the product. All attempts to effect the 1,5-HAT/5-exo-trig sequence from iodide 3, or its cycloheptene regiosomer (S3, Supplementary Materials), using a variety of conditions based on R3SnH and AIBN, returned complex product mixtures whose 1 H NMR spectra consistently showed resonances corresponding to RCH=CH2 [ 5.89-5.77 (1H, m), 5.01 (1H, dd, J 17.0, 2.0 Hz), 4.94 (1H, dd, J 10.5, 2.0 Hz)], indicating simple reduction of the C-I bond (→11) as a dominant pathway. A reaction with Bu3SnD was considered, in order to identify which of the 1,5-HAT or cyclisation steps was problematic, but this was discarded in light of the complex product profile and poor mass recovery; instead, an unambiguous route to the allylic radical 5 was sought. The ring-expanded derivatives 13 and 14 ( Figure  3) of (R)-carvone (and their enantiomers from (S)-carvone) are available on a multigram scale via efficient three-or four-step sequences. From these, enolate alkylation and carbonyl reduction would provide allylic alcohol 12 from which a variety of radical precursors could potentially be prepared. All attempts to effect the 1,5-HAT/5-exo-trig sequence from iodide 3, or its cycloheptene regiosomer (S3, Supplementary Materials), using a variety of conditions based on R 3 SnH and AIBN, returned complex product mixtures whose 1 H NMR spectra consistently showed resonances corresponding to RCH=CH 2 [δ H 5.89-5.77 (1H, m), 5.01 (1H, dd, J 17.0, 2.0 Hz), 4.94 (1H, dd, J 10.5, 2.0 Hz)], indicating simple reduction of the C-I bond (→11) as a dominant pathway. A reaction with Bu 3 SnD was considered, in order to identify which of the 1,5-HAT or cyclisation steps was problematic, but this was discarded in light of the complex product profile and poor mass recovery; instead, an unambiguous route to the allylic radical 5 was sought. The ring-expanded derivatives 13 and 14 ( Figure 3) of (R)-carvone (and their enantiomers from (S)-carvone) are available on a multigram scale via efficient three-or four-step sequences. From these, enolate alkylation and carbonyl reduction would provide allylic alcohol 12 from which a variety of radical precursors could potentially be prepared.  Cycloheptenone derivative 14 was prepared from (R)-carvone as described [42,43] and a sample was isomerised into the conjugated enone isomer 13 under basic conditions (Scheme 2). Direct 3-butenylation of similar ketones has been reported [44] but none of many combinations of base (LHMDS, NaH, t-BuONa, KHMDS, t-BuOK), solvent (THF, DMF, toluene, t-BuOH), additive (DMPU, HMPA), temperature (−78 °C to RT), and electrophile (4-bromobutene, 4-iodobutene) gave acceptable yields of the alkylated product from either cycloheptenone 13 or 14 (Supplementary Materials, Tables S1 and S2). A solution was found in Lautens' indirect method comprising alkylation with 1,4-dichlorobut-2-ene then Pd(0)-catalysed reductive isomerisation of the so-formed vinyl cyclopropane [45]. Applied to substrate 14, both stages of this method required much development. The alkylation step was best achieved using a mixed base system for the deprotonation [46] with 1,4-dibromobut-2-ene as an electrophile (Supplementary Materials, Table S3). The reductive rearrangement step was complicated by the presence of the endocyclic C=C bond that presumably stabilises the intermediate -allyl palladium enol(ate) 16 leading to O-alkylation product 17, sometimes as the major component in the crude product mixture. Eventually, it was found that replacing ammonium formate in the reported procedure with an excess of formic acid and triethylamine eliminated this side reaction and the desired butenylated product 18 was thereby obtained in ~65% yield from ketone 14 (Supplementary Materials, Table S4). Alkene isomerisation (→19) and Luche reduction completed the route to alcohol 12. The significant effort expended in achieving a reliable route to the alcohol was not rewarded by success in converting it into a suitable precursor to allylic radical 5. To summarise many attempts: O-thiocarbonyl derivatives showed a strong tendency to undergo [3,3]-shift leading to unreactive S-acyl isomers, and intermediates generated en route to the corresponding allylic halides or phenylselenides were generally unstable with respect to elimination leading to volatile cycloheptadienes. Cycloheptenone derivative 14 was prepared from (R)-carvone as described [42,43] and a sample was isomerised into the conjugated enone isomer 13 under basic conditions (Scheme 2). Direct 3-butenylation of similar ketones has been reported [44] but none of many combinations of base (LHMDS, NaH, t-BuONa, KHMDS, t-BuOK), solvent (THF, DMF, toluene, t-BuOH), additive (DMPU, HMPA), temperature (−78 • C to RT), and electrophile (4-bromobutene, 4-iodobutene) gave acceptable yields of the alkylated product from either cycloheptenone 13 or 14 (Supplementary Materials, Tables S1 and S2). A solution was found in Lautens' indirect method comprising alkylation with 1,4-dichlorobut-2-ene then Pd(0)-catalysed reductive isomerisation of the so-formed vinyl cyclopropane [45]. Applied to substrate 14, both stages of this method required much development. The alkylation step was best achieved using a mixed base system for the deprotonation [46] with 1,4-dibromobut-2-ene as an electrophile (Supplementary Materials, Table S3). The reductive rearrangement step was complicated by the presence of the endocyclic C=C bond that presumably stabilises the intermediate π-allyl palladium enol(ate) 16 leading to O-alkylation product 17, sometimes as the major component in the crude product mixture. Eventually, it was found that replacing ammonium formate in the reported procedure with an excess of formic acid and triethylamine eliminated this side reaction and the desired butenylated product 18 was thereby obtained in~65% yield from ketone 14 (Supplementary Materials, Table S4). Alkene isomerisation (→19) and Luche reduction completed the route to alcohol 12.  Cycloheptenone derivative 14 was prepared from (R)-carvone as described [42,43] and a sample was isomerised into the conjugated enone isomer 13 under basic conditions (Scheme 2). Direct 3-butenylation of similar ketones has been reported [44] but none of many combinations of base (LHMDS, NaH, t-BuONa, KHMDS, t-BuOK), solvent (THF, DMF, toluene, t-BuOH), additive (DMPU, HMPA), temperature (−78 °C to RT), and electrophile (4-bromobutene, 4-iodobutene) gave acceptable yields of the alkylated product from either cycloheptenone 13 or 14 (Supplementary Materials, Tables S1 and S2). A solution was found in Lautens' indirect method comprising alkylation with 1,4-dichlorobut-2-ene then Pd(0)-catalysed reductive isomerisation of the so-formed vinyl cyclopropane [45]. Applied to substrate 14, both stages of this method required much development. The alkylation step was best achieved using a mixed base system for the deprotonation [46] with 1,4-dibromobut-2-ene as an electrophile (Supplementary Materials, Table S3). The reductive rearrangement step was complicated by the presence of the endocyclic C=C bond that presumably stabilises the intermediate -allyl palladium enol(ate) 16 leading to O-alkylation product 17, sometimes as the major component in the crude product mixture. Eventually, it was found that replacing ammonium formate in the reported procedure with an excess of formic acid and triethylamine eliminated this side reaction and the desired butenylated product 18 was thereby obtained in ~65% yield from ketone 14 (Supplementary Materials, Table S4). Alkene isomerisation (→19) and Luche reduction completed the route to alcohol 12. The significant effort expended in achieving a reliable route to the alcohol was not rewarded by success in converting it into a suitable precursor to allylic radical 5. To summarise many attempts: O-thiocarbonyl derivatives showed a strong tendency to undergo [3,3]-shift leading to unreactive S-acyl isomers, and intermediates generated en route to the corresponding allylic halides or phenylselenides were generally unstable with respect to elimination leading to volatile cycloheptadienes. The significant effort expended in achieving a reliable route to the alcohol was not rewarded by success in converting it into a suitable precursor to allylic radical 5. To summarise many attempts: O-thiocarbonyl derivatives showed a strong tendency to undergo [3,3]-shift leading to unreactive S-acyl isomers, and intermediates generated en route to the corresponding allylic halides or phenylselenides were generally unstable with respect to elimination leading to volatile cycloheptadienes.
At this stage in the project, ketones 18 and 19 were identified as candidates for a reductive cyclisation of the derived ketyl radical anion (Step A, Figure 4) for which there is ample precedent [47][48][49][50][51][52]. In the context of the synthesis of africanol and its isomers, the related approaches of Tai [53], Cossy [44], and Piers [54] gave confidence that this could be a reasonably efficient process, with the stereochemical outcome being controllable to a certain extent by the choice of reaction conditions. Following this cyclisation, an ambitious endgame was planned (Step B, Figure 4) which, mechanistically, would involve (a) activation of the bridgehead alcohol to initiate its departure (→21), then (b) [1,2]-H shift followed by (c) hydride reduction at the less substituted terminus of the so-formed allylic carbocation 22 leading to the fully substituted alkene 23. No direct precedent for such a process could be found but the combination of a Brønsted or Lewis acid activator coupled with a trialkylsilane reducing agent [55][56][57][58][59] was envisaged to achieve the transformation. Notable, Carey's early work [57] with trifluoroacetic acid and trialkylsilanes showed that cations generated from 3 • -alcohols are able to undergo stereospecific rearrangement prior to reductive quenching; see also [60].
Molecules 2023, 28, x FOR PEER REVIEW 5 of 15 At this stage in the project, ketones 18 and 19 were identified as candidates for a reductive cyclisation of the derived ketyl radical anion (Step A, Figure 4) for which there is ample precedent [47][48][49][50][51][52]. In the context of the synthesis of africanol and its isomers, the related approaches of Tai [53], Cossy [44], and Piers [54] gave confidence that this could be a reasonably efficient process, with the stereochemical outcome being controllable to a certain extent by the choice of reaction conditions. Following this cyclisation, an ambitious endgame was planned (Step B, Figure 4) which, mechanistically, would involve (a) activation of the bridgehead alcohol to initiate its departure (→21), then (b) [1,2]-H shift followed by (c) hydride reduction at the less substituted terminus of the so-formed allylic carbocation 22 leading to the fully substituted alkene 23. No direct precedent for such a process could be found but the combination of a Brønsted or Lewis acid activator coupled with a trialkylsilane reducing agent [55][56][57][58][59] was envisaged to achieve the transformation. Notable, Carey's early work [57] with trifluoroacetic acid and trialkylsilanes showed that cations generated from 3°-alcohols are able to undergo stereospecific rearrangement prior to reductive quenching; see also [60]. Piers' experimental conditions [54] for reductive cyclisation were used as the starting point for the analogous reaction of ketone 18 submitted as a mixture of inseparable diastereomers. With minor adjustments to the published procedure (freshly prepared samarium diiodide, less HMPA, a more concentrated solution, shorter reaction time), unwanted side-reactions were minimised and just two major products were isolated (24 and 25, Scheme 3), their structures being confirmed by extensive NMR spectroscopic analysis including the key NOE correlations shown ( Figure 5; Supplementary Materials, pp. S11-S14). The reaction showed high stereocontrol, with one major product arising from each diastereomer of the starting ketone; notably, both products had the same trans-relative disposition of methyl and hydroxyl groups with respect to the fixed isopropenyl substituent. When the separated alcohol diastereomers were treated with boron trifluoride etherate at low temperatures in the presence of an excess of triethylsilane, the outcomes differed dramatically. In the case of trans-fused bicyclic alcohol 24, the reaction proceeded cleanly to deliver a single hydrocarbon 26, with the merely moderate isolated yield reflecting the high volatility of the product. In contrast, the analogous reaction with cis-fused bicyclic alcohol 25 gave no clear products. From these results, it seems reasonable to propose that the departure of the Lewis acid-complexed hydroxyl group in substrate 24 benefits from participation by the antiperiplanar C-H bond leading directly to a relatively long-lived allylic cation; for 25, no such participation can occur so the localised 3°-carbocation must form fully from which a variety of uncontrolled reactions compete to the exclusion of clean [1,2]-H shift. Piers' experimental conditions [54] for reductive cyclisation were used as the starting point for the analogous reaction of ketone 18 submitted as a mixture of inseparable diastereomers. With minor adjustments to the published procedure (freshly prepared samarium diiodide, less HMPA, a more concentrated solution, shorter reaction time), unwanted sidereactions were minimised and just two major products were isolated (24 and 25, Scheme 3), their structures being confirmed by extensive NMR spectroscopic analysis including the key NOE correlations shown ( Figure 5; Supplementary Materials, pp. S11-S14). The reaction showed high stereocontrol, with one major product arising from each diastereomer of the starting ketone; notably, both products had the same trans-relative disposition of methyl and hydroxyl groups with respect to the fixed isopropenyl substituent. When the separated alcohol diastereomers were treated with boron trifluoride etherate at low temperatures in the presence of an excess of triethylsilane, the outcomes differed dramatically. In the case of trans-fused bicyclic alcohol 24, the reaction proceeded cleanly to deliver a single hydrocarbon 26, with the merely moderate isolated yield reflecting the high volatility of the product. In contrast, the analogous reaction with cis-fused bicyclic alcohol 25 gave no clear products. From these results, it seems reasonable to propose that the departure of the Lewis acid-complexed hydroxyl group in substrate 24 benefits from participation by the antiperiplanar C-H bond leading directly to a relatively long-lived allylic cation; for 25, no such participation can occur so the localised 3 • -carbocation must form fully from which a variety of uncontrolled reactions compete to the exclusion of clean [1,2]-H shift.
The isolated hydrocarbon 26 is assigned as 5-epi-α-bulnesene on the basis of a full NMR spectroscopic analysis; however, very limited NMR data are reported for the epimers of α-bulnesene with which to corroborate the assigned structure. For 5-epi-α-bulnesene, 1 H NMR resonances are available [26] for just =CH 2 (δ 4.66) and the methyls at C-13 (δ 1.72), C-14 (δ 0.95), and C-15 (δ 1.67); the values obtained for compound 26 are, respectively: δ 4.73/4.70, 1.75, 0.98, and 1.58 with the resonance for CH 3 -15 differing by −0.09 ppm, out of line with the others that differ consistently by +0.03-0.05 ppm, potentially attributable to referencing methods. We considered a number of alternative structures that could reasonably arise from a more complex reductive rearrangement but none of these fit the COSY and HMBC data (Tables 1 and 2; Supplementary Materials, pp. S15-S17) as well as the assigned structure; indeed, some of these are reported [61] and can be confidently excluded. Furthermore, apart from the C-7 resonance in the 13 C NMR spectrum, all the 13 C chemical shifts are within~3 ppm of the values for authentic α-bulnesene ( Figure 6); the aberrant C-7 resonance (43.5 vs. 51.0 for α-bulnesene in CDCl 3 ; 43.8 vs. 51.2 for αbulnesene in C 6 D 6 ) is, however, not unreasonable when compared with that reported [62] for 4,5-di-epi-α-bulnesene (δ 43.5 in C 6 D 6 ). The isolated hydrocarbon 26 is assigned as 5-epi--bulnesene on the basis of a ful NMR spectroscopic analysis; however, very limited NMR data are reported for the epi mers of -bulnesene with which to corroborate the assigned structure. For 5-epi- bulnesene, 1 H NMR resonances are available [26] for just =CH2 ( 4.66) and the methyls a C-13 ( 1.72), C-14 ( 0.95), and C-15 ( 1.67); the values obtained for compound 26 are respectively:  4.73/4.70, 1.75, 0.98, and 1.58 with the resonance for CH3-15 differing b −0.09 ppm, out of line with the others that differ consistently by +0.03-0.05 ppm, poten tially attributable to referencing methods. We considered a number of alternative struc tures that could reasonably arise from a more complex reductive rearrangement but non of these fit the COSY and HMBC data (Tables 1 and 2; Supplementary Materials, pp. S15 S17) as well as the assigned structure; indeed, some of these are reported [61] and can b confidently excluded. Furthermore, apart from the C-7 resonance in the 13 C NMR spec trum, all the 13 C chemical shifts are within ~3 ppm of the values for authentic -bulnesen ( Figure 6); the aberrant C-7 resonance (43.5 vs. 51.0 for -bulnesene in CDCl3; 43.8 vs. 51. for -bulnesene in C6D6) is, however, not unreasonable when compared with that reported [62] for 4,5-di-epi--bulnesene ( 43.5 in C6D6).  The isolated hydrocarbon 26 is assigned as 5-epi--bulnesene on the basis NMR spectroscopic analysis; however, very limited NMR data are reported for mers of -bulnesene with which to corroborate the assigned structure. For bulnesene, 1 H NMR resonances are available [26] for just =CH2 ( 4.66) and the m C-13 ( 1.72), C-14 ( 0.95), and C-15 ( 1.67); the values obtained for compound respectively:  4.73/4.70, 1.75, 0.98, and 1.58 with the resonance for CH3-15 diff −0.09 ppm, out of line with the others that differ consistently by +0.03-0.05 ppm tially attributable to referencing methods. We considered a number of alternativ   H3-15 H2-2, H-5 1 Ambiguous correlation between overlapping multiplets.

General Methods
All solvents for anhydrous reactions were obtained dry from Grubbs solvent dis penser units after being passed through an activated alumina column under argon. TH was additionally distilled from sodium/benzophenone ketyl under argon. Commerciall available reagents were, in general, used as supplied; amines and dipolar aprotic solvent (HMPA and NMP) were purified by standard methods before use. "Petrol" refers to th fraction of light petroleum ether boiling in the range of 30-40 °C; "ether" refers to diethy ether. All reactions were carried out in oven-dried glassware and under an inert atmos phere (N2 or Ar as specified). Thin layer chromatography (TLC) was carried out usin Merck aluminium-backed DC60 F254 0.2 mm precoated plates. Spots were then visualised by the quenching of ultraviolet light fluorescence (max 254 nm) and then stained and heated with either anisaldehyde or KMnO4 solutions as appropriate. Retention factors (R are reported along with the solvent system used in parentheses. Flash column

General Methods
All solvents for anhydrous reactions were obtained dry from Grubbs solvent dispenser units after being passed through an activated alumina column under argon. THF was additionally distilled from sodium/benzophenone ketyl under argon. Commercially available reagents were, in general, used as supplied; amines and dipolar aprotic solvents (HMPA and NMP) were purified by standard methods before use. "Petrol" refers to the fraction of light petroleum ether boiling in the range of 30-40 • C; "ether" refers to diethyl ether. All reactions were carried out in oven-dried glassware and under an inert atmosphere (N 2 or Ar as specified). Thin layer chromatography (TLC) was carried out using Merck aluminium-backed DC60 F254 0.2 mm precoated plates. Spots were then visualised by the quenching of ultraviolet light fluorescence (λ max 254 nm) and then stained and heated with either anisaldehyde or KMnO 4 solutions as appropriate. Retention factors (R f ) are reported along with the solvent system used in parentheses. Flash column chromatography was performed using Merck 60 silica gel (particle size 40-63 µm), and the solvent system used is reported in parentheses. Infrared spectra were recorded using a Bruker Tensor 27 FT-IR fitted with a diamond ATR module. Absorption maxima (ν max ) are reported in wavenumbers (cm −1 ) and are described as strong (s), medium (m), weak (w) or broad (br). Proton ( 1 H) and carbon-13 ( 13 C) NMR spectra were recorded on Bruker NEO-600 or AVIIIHD-400 spectrometers. Chemical shifts (δ H or δ C ) are reported in parts per mil-  26. Peak multiplicities are described as singlet (s), doublet (d), triplet (t), quartet (q), quintet (quin), multiplet (m), and broad (br) or a combination thereof. Coupling constants (J) are rounded to the nearest 0.5 Hz. Assignments are made on the basis of chemical shifts, integrations, and coupling constants, using COSY, HSQC, HMBC, and NOESY experiments where appropriate. High-resolution mass spectra (HRMS) were recorded by the staff at the Chemistry Research Laboratory (University of Oxford) using a Bruker Daltonics MicroTOF spectrometer; mass-to-charge ratios (m/z) are reported in Daltons. Melting points were recorded on a Griffin MFB-700-010U melting point apparatus and are uncorrected.  A solution of [4-(trimethylsilyl)but-3-yn-1-yl]magnesium bromide in THF was prepared by adding (4-bromobut-1-yn-1-yl)trimethylsilane (156 mg, 0.760 mmol) into a stirred suspension of Mg (184 mg, 7.57 mmol) in dry THF (10 mL). After formation of the grey Grignard solution was complete, it was added into a −30 • C solution of enol triflate 9 (113 mg, 0.379 mmol), Fe(acac) 3 (7.0 mg, 0.02 mmol), and NMP (0.30 mL, 3.1 mmol) in dry THF (10 mL) under N 2 , leading to an immediate colour change from orange to black. The mixture was stirred for 1 h at −30 • C and was then quenched with saturated aqueous NH 4 Cl solution (10 mL). The aqueous phase was extracted with ether (3 × 10 mL) then the combined organic extracts were dried (Na 2 SO 4 ), filtered, and concentrated. The residue was dissolved in dry THF (5 mL) then TBAF (0.19 mL, 1.0 M in THF, 0.19 mmol) was added and the mixture stirred under N 2 for 1 h at RT. Saturated aqueous NH 4 Cl solution (10 mL) was added and the aqueous layer was extracted with ether (3 × 10 mL). The combined organic extracts were dried (MgSO 4 ), filtered, and concentrated. The residue was purified by column chromatography (pentane) to give the title compound 10 as a colourless oil (19 mg, 25%). This volatile compound was used directly in the next step.   DBU (0.40 mL, 2.7 mmol) was added to a stirred solution of cycloheptenone derivative 14 (420 mg, 2.56 mmol) in dichloromethane (13 mL) at RT under N 2 . The mixture was stirred for 3 h and then quenched with the addition of hydrochloric acid (5 mL, 1.0 M). The layers were separated, and the aqueous layer was extracted with dichloromethane (2 × 10 mL) then the combined organic portions were washed with brine (10 mL), dried (MgSO 4 ), filtered, and concentrated to yield the title compound 13 (400 mg, 95%), a colourless oil, that was sufficiently pure to use in further steps. A sample was purified for analytical purposes by column chromatography (pentane/ether, 9:1). The analytical data are as reported in [63]. Potassium tert-butoxide (4.0 mL, 1.0 M in THF, 4.0 mmol) was added to a RT suspension of NaH (267 mg, 60% in mineral oil, 6.68 mmol) in dry toluene (80 mL) under Ar and the mixture was stirred for 1 h. A solution of ketone 14 (550 mg, 3.35 mmol) in dry toluene (10 mL) was added via syringe, stirring was continued for 1 h then the enolate solution was cooled to −78 • C and a solution of (E)-1,4-dibromo-2-butene (2.86 g, 13.4 mmol) in dry toluene (20 mL) was added dropwise over 10 min. The mixture was warmed slowly to RT and stirred for 2 h then quenched with saturated aqueous NH 4 Cl solution (20 mL) and extracted with ether (3 × 20 mL). The combined extracts were washed with brine (20 mL), dried (MgSO 4 ), filtered, and concentrated to give a residue that was purified by chromatography (pentane/ethyl acetate, 20:1), affording the product 15 as a yellow oil and as a pair of unassigned diastereomers (527 mg, 73%, dr = 1:1). R     A solution of potassium tert-butoxide (0.030 mL, 1.0 M in THF, 0.030 mmol) was added to a RT solution of ketone 18 (22 mg, 0.10 mmol) in dry toluene (2 mL) under N 2 . The mixture was heated to 90 • C and stirred for 4 h. The resulting orange solution was cooled to RT and quenched with saturated aqueous NH 4 Cl solution (5 mL) then extracted with ether (3 × 5 mL). The combined organic phases were dried (Na 2 SO 4 ), filtered, and concentrated. The residue was purified by chromatography (pentane/ethyl acetate, 30:1) to give the ketone 19 (7.0 mg, 32%) as a colourless oil and recovered ketone 18 ( A solution of BF 3 ·OEt 2 (0.0152 mL, 0.120 mmol) in dry dichloromethane (1.52 mL) was added dropwise over 5 min to a −78 • C solution of alcohol 24 (39.0 mg, 0.177 mmol) and triethylsilane (0.168 mL, 1.05 mmol) in dry dichloromethane (2 mL) under N 2 . The resulting mixture was allowed to warm slowly to 0 • C and was then stirred for 1 h. Saturated aqueous NaHCO 3 solution (10 mL) was added, and the mixture was extracted with ether (3 × 10 mL). The combined organic extracts were dried (MgSO 4 ), filtered, and concentrated carefully under vacuum. The residue was purified by chromatography (pentane) to give the title compound 26 (19.

Conclusions
This study set out to develop a short synthesis of both enantiomers of α-bulnesene in order to investigate their relative reactivity towards our local library of P450 BM3 enzymes and the fragrance properties of oxidised derivatives in both enantiomeric series. The initially planned 1,5-HAT/cyclisation strategy was not productive but led to the exploration of a related ketyl radical anion cyclisation. During this project, optimised conditions were developed for overall butenylation of enone 14; a modified reductive cyclisation protocol led to increased efficiency and stereoselectivity compared with previously reported close precedents, and a new reductive rearrangement step completed the synthesis not of αbulnesene itself but of the 5-epi-isomer. The overall route ( Figure 7A) from carvone is short (seven steps) and sufficiently efficient (~15%) for the project's purposes. Current work is exploring methods [64][65][66] to produce the "missing stereoisomer" 27 projected to lead to the natural product ( Figure 7B) and, from (S)-carvone, its enantiomer. Supplementary Materials: The following supporting information can be downloaded at: www.mdpi.com/xxx/s1. NMR spectra for compounds in Schemes 1-3, reference NMR data for bulnesene 1, procedures and characterisation data for S1-S3, optimisation tables for the formation of butenyl ketone 18 (PDF).